Structures useful in electron beam lithography

ABSTRACT

A method for forming a mask assembly for use in lithography, typically electron-beam lithography, first forms in a substrate one half of a plurality of opening therethrough and then fills the openings with a removable fill material. Thereafter are formed the other half of the openings which are then filled with the removable fill material. After all the openings have been formed and filled, a support membrane is formed over the substrate and covers the filled windows. A mask layer is then formed over the membrane and patterned. The fill is then removed from all of the windows.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. Ser. No. entitled “Methods For Fabricating Stencil Masks”, which has a common assignee and one common inventor and is being filed concurrently with this application.

FIELD OF THE INVENTION

[0002] This invention relates to lithography, and more particularly, to masks used in lithography and mask support structures and their preparation.

BACKGROUND OF THE INVENTION

[0003] As the size of electronic components used in integrated circuits has shrunk and their density in circuits increased, there has been growing interest in is electron beam lithography. In electron beam lithography, an electron beam is used to write patterns on an electron beam resist layer formed over a top surface of the semiconductor wafer that is the workpiece for the electron beam lithography. A particular form of electron beam lithography of special interest for the invention is described as Scattering With Angular Limiting Projection Electron Beam Lithography (“Scalpel”).

[0004] Scalpel is an electron beam lithography technique that typically has employed masks in which the mask used for control in the irradiation of the workpiece has been formed as a thin patterned coating supported on a thin membrane. Scalpel has been described in various publications.

[0005] The requirements for such a membrane-mask combination are quite demanding. Typically, it needs to be planar, relatively easy to make, and comparatively rugged. A particular problem has been the forming of the membrane that is to support the coating in which the mask pattern is formed. The membrane needs to be of relatively large area, typically at least several inches on a side, to be practical for use in large scale manufacture. It needs also to be very thin, typically no thicker than about 1000 Angstroms in thickness, to permit electrons that pass through the mask to penetrate it easily without excessive scattering or experiencing excessive loss in electron beam energy. Moreover, it also needs to remain planar with inappreciable sag in use so that it continues to support the mask uniformly over its entire area.

[0006] To insure that the membrane remains sag-free, it is generally the practice to suspend the membrane by a substrate that provides a suitable underlying support grillage, typically consisting of major struts and minor struts (ribs).

[0007] Hitherto, for forming the grillage for supporting the membrane that supports the mask coating, a layer of silicon nitrite was deposited over the top surface of a silicon wafer and then the silicon wafer was etched to leave a portion thereof to provide the supporting grillage for the nitride coating that was to serve as the membrane. However, this has proven difficult.

[0008] One problem with this approach has been the difficulty of maintaining, during the shaping of the silicon wafer, the mechanical stability of the grillage left to support nitride the membrane. The physical dimensions of commercially available silicon wafers limits formation of thin struts needed.

[0009] The present invention involves a better way to prepare a mask useful in electron beam lithography.

SUMMARY OF THE INVENTION

[0010] In a first aspect the invention is a method for forming a mask assembly for use in lithography. The method comprises the steps of: forming a support structure that comprises a substrate that includes a plurality of windows filled with a temporary fill; forming over the filled-windowed substrate a mask; and removing the temporary fill.

[0011] In a second aspect the present invention is a method for forming a mask assembly for use in lithography. The method comprises the steps of: forming a support structure that comprises a substrate that includes a plurality of windows filled with a temporary fill; forming over the filled-windowed substrate a membrane layer for supporting the mask layer; forming a mask layer over the membrane layer; and removing the temporary fill.

[0012] In a third aspect the invention is a method of forming a mask assembly that comprises the steps of: forming in a substrate a support structure, which includes major and minor struts that define an array of windows in a two-dimensional array of rows and columns, by successive rounds of cutting in the substrate a fraction of the total window area to be formed; filling such fraction of windows with temporary fill before the succeeding round of cutting and filling until all the window areas are cut and filled; forming a membrane layer over a top surface of the support structure; forming a mask layer over the membrane layer; and removing the fill from the windows.

[0013] In a fourth aspect the present invention is a method of forming a mask assembly for use in electron beam lithography that comprises the steps of: forming in a substrate a first set of spaced-apart windows; filling the windows with a temporary fill; forming in the substrate a second set of windows in the spaces between the first set of windows for forming with the first set a two-dimensional array of windows arranged in row and columns; filling the second set of windows with a temporary fill; depositing over the filled-windowed substrate a layer suitable for supporting a mask; depositing over the last-mentioned layer a layer suitable for providing a mask; patterning the last-mentioned layer to form a mask, and removing the temporary fill from the windows, whereby the mask layer is free of underlying substrate.

[0014] In a fifth aspect the present invention is a method for forming a mask assembly comprising the steps of: forming by use of a mold a support structure that defines an array of windows arranged in rows and columns; filling the openings with a temporary fill; forming over the support structure a membrane layer; forming over the membrane layer a patterned mask; and removing the temporary fill.

[0015] In a sixth aspect the invention is a method of forming a mask support structure that comprises the steps of: forming in a substrate a first set of spaced apart windows; filling the first set of windows with a temporary fill; forming in the substrate a second set of windows located in portions of the substrate between the first set of filled windows; and filling the second set of windows with a temporary fill.

[0016] In a seventh aspect the invention is a method of forming a mask support structure that comprises the steps of: placing in a mold which is shaped to facilitate the formation of a support structure a plurality of parallel minor struts; and forming in the mold a mask support structure that comprises a frame and plurality of major struts that are orthogonal and attached to the minor struts with the major and minor struts defining a plurality of windows arranged in a two dimensional array of rows and columns.

[0017] From a product aspect, the invention is the product of the various methods described.

[0018] The invention will be better understood from the following more detailed description in conjunction with the accompanying drawing and claims.

BRIEF DESCRIPTION OF THE DRAWING

[0019]FIG. 1. shows a top view of a mask support structure in accordance with the invention suitable for use in electron beam lithography;

[0020]FIG. 2 shows a cross-sectional view taken along a dashed line 2-2 of FIG. 1 with a membrane layer over the mask support structure;

[0021]FIGS. 3, 4, 5, and 6 show steps in the fabrication of the mask support structure of FIG. 1;

[0022]FIG. 7 shows a cross sectional view taken along a dashed line 7-7 of FIG. 6;

[0023]FIG. 8 shows the section of FIG. 7 after there has been deposited a membrane over a top surface thereof;

[0024]FIG. 9 shows the section of FIG. 8 after there has been deposited over the membrane a layer that will be patterned to serve as the mask for electron-beam lithography;

[0025]FIG. 10 shows the section of FIG. 8 after the fill has been removed from the windows;

[0026]FIGS. 11-14 show cross sectional views that illustrate an alternative method for providing the membrane and mask layers over a mask support structure of the kind shown in FIG. 7; and

[0027]FIGS. 15, 16, and 17 shown a top view, a first cross-sectional view through a dashed line 51-51 of FIG. 15, and a second cross-sectional view through a dashed line 52-52 of FIG. 15 of a mold useful in the production of a mask support structure in accordance with another embodiment of the invention;

[0028]FIG. 18 shows the resulting mask support structure after it is removed from the mold;

[0029]FIG. 19 shows a three-dimensional view of a mask support structure in an early stage of fabrication in accordance with another embodiment of the present invention;

[0030]FIG. 20 shows the mask support structure of FIG. 19 at a succeeding stage of fabrication; and

[0031]FIG. 21 shows a cross-sectional view taken along a dashed line 2-2 of FIG. 1 with a stencil mask over the mask support structure;

[0032] The drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Referring now to FIGS. 1 and 2, there is shown a mask support structure 10 for use in electron beam-lithography in accordance with the invention with FIG. 1 showing a top view and FIG. 2 showing a cross-sectioned view through a dashed line 2-2 of FIG. 1. Mask support structure 10 comprises a substrate 12 having an outer frame portion 12 a, advantageously of square shape, major struts 12 b, and minor struts 12 c. The outer frame portion 12 a can be any shape, for example, circular. The major struts are vertical regions and the minor struts are horizontal regions. Major struts 12 b and minor struts 12 c are orthogonal to each other and define a plurality of windows (openings) 18, arranged in an array of rows and columns. The major struts 12 b are at least several times thicker than the minor struts 12 c.

[0034] As seen in the cross-sectioned view of FIG. 2, over a top surface of the mask support structure is a thin membrane layer 20 that is to support a layer (not shown) that will have been patterned to serve as a mask layer. Mask support structure 10 with the membrane layer 20 on top thereof and a patterned mask layer 28 (shown in FIGS. 9-14) over the membrane layer 20 may be denoted as a mask assembly. The mask layer typically is of a material and thickness to scatter electrons to sufficient angles that it intercepts where unpatterned to be useful in electron beam lithography, but allows electrons to pass through the openings in the pattern little impeded. The membrane 20 needs be of a material and thickness adequate to support the mask layer without sagging but to little affect the passage of electrons therethrough.

[0035] In accordance with the invention, the mask support structure assembly 10 is prepared in a novel fashion. To this end, as is shown in FIG. 3 there is provided a substrate 12 of a suitable material and advantageously a ceramic, such as aluminum oxide or silicon carbide, of sufficient thickness to be rigid and self supporting, in which are shown by solid lines a first set of windows 18 a that are cut in a first round of cutting. In this first round, there are cut one half of the total number of windows to be cut. Advantageously all the windows to be cut are to form a two-dimensional array in rows and columns, as shown in FIG. 1, and in this first round, there are cut alternate windows 18 a in each row and column as is denoted by the solid lines. Also shown in FIG. 3 by dashed lines are the regions where windows 18 b of the second set that are to be cut in the second round of cuttings. These will form with the first set 18 a the two-dimensional array of windows. In one example the windows 18 a and 18 b are each about 12 millimeters by 1.1 millimeters, and are formed by laser cutting.

[0036] After the first set of half of the window pattern has been cut out to form windows 18 a, these windows 18 a are temporarily filled with a suitable material that can later be conveniently introduced and later conveniently removed, advantageously an epoxy, silicon oxide, a polymer, or a metal such as aluminum. The result is seen in FIG. 4 where the first set of filled window regions 18 a is shown stippled.

[0037] Next there are cut out the remaining windows 18 b of the second set, as is shown in FIG. 5, typically in the same manner as before.

[0038] Then the second set of windows 18 b are filled typically in the manner used previously to fill the first set. The result is shown in a top view in FIG. 6 and in a cross-sectional view through a dashed line 7-7 of FIG. 6 in FIG. 7. All of the window regions 18 a and 18 b are shown filled, as is indicated by the stippling in both windows 18 a and 18 b.

[0039] Next, the opposite major surfaces of windowed substrate 12 are ground and polished to provide a mask support structure with smooth, planar, and parallel opposite major surfaces. This can be done, for example, by chemical mechanical polishing (CMP).

[0040] As mentioned earlier, there are various ways consistent with the invention in which the membrane layer 20 that will support a mask layer 28 can be provided on the mask support structure 10.

[0041] In one method there is deposited, typically by chemical vapor deposition, over a major surface of the mask support structure 10 a layer of the material that will form a membrane layer 20 for supporting the mask layer 28 (shown in FIGS. 9-14), as is shown in the cross-section view of FIG. 8. Suitable membrane materials include silicon nitride, silicon, silicon carbide, diamond, and aluminum oxide. The membrane should be of a thickness sufficient to adequately support the mask layer but to little impede the passage therethrough of electrons that pass through the patterned mask layer. A thickness of 1000 Angstroms or less should be suitable.

[0042] Next, as is shown in the cross-sectional view of FIG. 9, there is deposited over a top surface of the membrane layer 20 a layer that will be patterned to serve as a mask layer 28. The mask layer 28 typically is of a material and thickness to scatter electrons to sufficient angles that it intercepts where unpatterned to be useful in electron beam lithography, but allows electrons to pass through the openings in the pattern little impeded. Examples include tungsten, and tantalum silicon nitride, about 300 Angstroms thick.

[0043] Next, the mask layer 28 is patterned appropriately in any suitable manner. Typically this is done by first coating it with a standard electron beam resist, and then patterning the resist coating in the desired mask pattern with an electron beam. Then this mask pattern is transferred in the usual manner from the resist coating into the mask layer 28, after which the resist coating is removed to leave a patterned mask layer 28 as a coating on the membrane.

[0044] Now, there is removed the fill material from the windows 18 a and 18 b to open them. The result is shown in FIG. 10 in which the fill has been removed from the windows 18 a and 18 b formed between the major and minor struts 12 b and 12 c and the mask layer 28 has been patterned to serve as a mask. Advantageously the fill material was suitably chosen to permit its easy removal, as by wet chemical etching without damage to the mask. In some instances, it may be possible to remove the fill in the same process used to remove the unneeded portions of the mask layer. In specific cases, the fill may have been removed just before the mask layer was patterned, unless the additional support the fill provides was still needed during the patterning.

[0045] This results in a mask support assembly with a mask layer 28 thereon.

[0046] It can be appreciated that a fraction different from one half, for example a third, appropriately chosen to maintain adequate rigidity, of the total number of windows to be cut, can be cut (formed) and filled in separate rounds during the formation of such windows Additionally, it can be appreciated that rather than a fraction of the total number of windows to be formed can be cut and filled in each round, a fraction of the area of each window to be formed can be instead cut and filled in successive rounds, to maintain the desired strength of the substrate during the window-cutting steps.

[0047] As was also mentioned above, various other methods consistent with the invention can be used to provide a membrane layer 20 for supporting the mask layer 28 on the mask support structure 10 shown in FIG. 7. For example, in another method there is prepared a suitable second substrate 30, for example, of silicon, typically of the same size and shape as the substrate 12. As shown in FIG. 11, one major surface of the substrate 30 is covered in turn with an underlying layer of a thickness and material suited for use as the mask layer 28 and an overlying layer of a size and material suitable for use as the membrane layer 20.

[0048] Then, as is shown in FIG. 12, the second substrate 30 is bonded by way of its membrane layer 20 to a major surface of the filled-window mask support structure 10 of FIG. 7 by any suitable means, as by plasma activation bonding (not shown) or the use of an adhesive layer (not shown).

[0049] Then there is removed the second substrate portion 30, as by etching, to leave the structure shown in FIG. 13, in which mask layer 28 is exposed, supported on the first substrate 12 by way of the membrane layer 20.

[0050] There can now be patterned the exposed mask layer 28, in the manner previously described.

[0051] There can now be removed the fill in the supporting grillage to leave the structure shown in FIG. 14, which is the structure desired for mask assembly.

[0052] To facilitate the removal of the second substrate 30, it can be ion-implanted before bonding to the mask support structure 10, to form a defect-rich buried layer in its interior. The ions could, for example, be hydrogen or oxygen. This makes it is easy to cleave the substrate 30 along such defect-rich layer. The cleaving of the substrate 30 can appreciably thin the substrate 30 and thus facilitates its subsequent complete removal to expose the layer that is to form the mask layer 28. The dashed line 36 across substrate 30 in FIG. 12 illustrates the location of such an optional ion-implanted layer within substrate 30.

[0053] Referring now to FIGS. 15, 16, and 17, there are shown a top view (FIG. 15), a first cross-sectional view (FIG. 16) through a dashed line 16-16 of FIG. 15, and a second cross-sectional view (FIG. 17) through a dashed line 17-17 of FIG. 15 of a mold 50 useful for forming a support structure in accordance with another embodiment of the invention. The mold includes wide openings for defining the major struts and narrower openings (grooves) for defining the minor struts. A plurality of parallel minor struts (elongated strips) 54 have been placed in the grooves of the mold 50. When mold 50 is filled with a molding material, the molding material attachs itself to minor struts 54. Grooves in the mold 50 have a depth (height) of “d” (See FIGS. 16 and 17). Minor struts 54, which typically has a depth less than “d”, can optionally be of the same depth (height) d as the grooves in the mold 50. The material for the minor struts 54 and the mold-filling material can be the same or different materials. After the fill material hardens a mask support structure 100 (see FIG. 18) is formed in the mold 50.

[0054]FIG. 18 shows the mask support structure 100 after there has been removed from the mold 50. Mask support structure 100 has a frame 56 comprising major struts 56 a and minor struts 54 that define rows and columns of an array of openings (windows) 58. Mask support structure 100 is similar to mask support structure 10 of FIG. 1. It is then advantageously filled with a temporary fill (not shown) and then opposite major surfaces are planarized. Mask support structure 100 is then used as a support for a membrane layer and a mask layer in essentially the same manner as explained previously for support structure 10 of FIG. 1. Still another method of forming a suitable mask support structure for use in the invention is described below with reference to FIGS. 19 and 20.

[0055] Referring now to FIG. 19, there is shown a three-dimensional view of a frame 60 which has a plurality of spaced apart large rectangular opening 62 formed therethrough. Only three openings 62 are shown. Frame 60 is useful for forming a support structure in accordance with an other embodiment of the invention. A plurality of parallel grooves 64 are formed that extend across the frame including the regions between openings 62. Grooves 64 can optionally be of the same depth (height) as the openings 62.

[0056] Referring now to FIG. 20, a separate minor strut 66 is inserted into each of the groves 64 to divide the large openings and to form a mask support structure 1000 in accordance with an other embodiment of the invention. Mask support structure 1000 of FIG. 20 is similar to support structure 10 of FIG. 1 and to mask support structure 100 of FIG. 18 and can be used in the manner previously described for such mask support structures. It is then advantageously filled with a temporary fill (not shown) and then opposite major surfaces thereof are planarized. Mask support structure 1000 is then used as a support for a membrane layer and a mask layer in essentially the same manner as explained previously for mask support structure 10 of FIG. 1.

[0057] Referring now to FIG. 21, there is shown a cross-sectional view of the mask support structure 10 of FIG. 1 with a stencil mask 21 over a top surface of support structure 10. Stencil mask 21 serves same the purpose as membrane layer 20 (shown in FIG. 2) and the mask layer 28 shown in FIGS. 9-14 and can be substituted for membrane layer 20 and mask layer 28. Stencil mask 21 has formed therethrough a plurality of openings 21 a which define a pattern. Stencil mask 21 can be used with the mask support structure 100 of FIG. 18 and with the mask support structure 1000 of FIG. 19.

[0058] It can be appreciated that the principles of the invention are essentially independent of the specific materials employed and of the specific dimensions described. For example, subsequent developments in materials and in cutting and filling techniques will likely affect what is preferred in the future. Still further, it should be apparent that in the embodiment involving successive rounds of cutting and filling that it may not be necessary to interleave all of the window opening of different rounds since it should be feasible in some instances to include in the same cutting round pairs or more of adjacent windows of the window pattern desired. 

What is claimed is:
 1. A method for forming a mask assembly for use in lithography comprising the steps of: forming a support structure that comprises a substrate that includes a plurality of windows filled with a temporary fill; forming over the filled-windowed substrate a mask; and removing the temporary fill.
 2. The method of claim 1 wherein the mask comprises a membrane layer covered by a mask layer.
 3. The method of claim 1 wherein the mask is a stencil mask.
 4. A method for forming a mask assembly for use in lithography comprising the steps of: forming a support structure that comprises a substrate that includes a plurality of windows filled with a temporary fill; forming over the filled-windowed substrate a membrane layer for supporting the mask layer; and forming a mask layer over the membrane layer; and removing the temporary fill.
 5. The method of claim 4 in which the support structure is formed by the steps of: forming in a substrate a first set of spaced apart windows; filling the first set of windows with a temporary fill; forming in the substrate a second set of windows located in portions of the substrate between the first set of filled windows; and filling the second set of windows with a temporary fill.
 6. The method of claim 5 in which the first partial set of windows is approximately one half of the total number of windows to be formed and the second set includes the remaining windows to be filled.
 7. The method of claim 6 in which the windows are in a two dimensional array of rows and columns and in which the first set consists of alternate windows in each row and column.
 8. A method of forming a mask assembly comprising the steps of: forming in a substrate a support structure, which includes major and minor struts that define an array of windows in a two-dimensional array of rows and columns, by successive rounds of cutting in the substrate a fraction of the total window area to be formed; filling such fraction of windows with temporary fill before the succeeding round of cutting and filling until all the window areas are cut and filled; forming a membrane layer over a top surface of the support structure; forming a mask layer over the membrane layer; and removing the fill from the windows.
 9. The method of claim 8 in which the first round of cutting involves cutting approximately one half of the windows to be cut and a second round involves the remainder.
 10. The method of claim 9 in which the first round of cutting is of alternate windows in each row and column.
 11. The method of claim 4 in which the support structure is formed by the steps of: placing in a mold which is shaped to facilitate the formation of a support structure a plurality of parallel minor struts; and forming in the mold a support structure that comprises a frame and plurality of major struts that are orthogonal and attached to the minor struts with the major and minor struts defining a plurality of windows arranged in a two dimensional array of rows and columns.
 12. The method of claim 11 further comprising the step of removing the support structure from the mold.
 13. The method of claim 4 in which the forming over the filled-windowed substrate of the membrane and mask layers includes the steps of: forming over a surface of a second substrate in turn a layer suitable for the mask and a layer suitable for the membrane; bonding the membrane layer of the second substrate to the first-mentioned substrate in a manner to expose the second substrate, and removing selectively the second substrate to expose the mask layer.
 14. The method of claim 13 in which the second substrate is first implanted with ions to create in its interior an ion-implanted region and the second substrate is removed in part by cleaving along the ion-implanted region.
 15. The method of claim 7 in which the forming over the filled-windowed substrate includes the steps of: forming over a second substrate a mask layer and a membrane layer; bonding the membrane layer to the filed-windowed substrate and leaving exposed the second substrate; selectively removing the second substrate to expose the mask layer.
 16. A method of forming a mask assembly for use in electron beam lithography comprising the steps of: forming in a substrate a first set of spaced-apart windows; filling the windows with a temporary fill; forming in the substrate a second set of windows in the spaces between the first set of windows for forming with the first set a two-dimensional array of windows arranged in row and columns; filling the second set of windows with a temporary fill; depositing over the filled-windowed substrate a layer suitable for supporting a mask; depositing over the last-mentioned layer a layer suitable for providing a mask; patterning the last-mentioned layer to form a mask, and removing the temporary fill from the windows, whereby the mask layer is free of underlying substrate.
 17. The method of claim 16 in which the substrate is chosen from the group consisting of aluminum oxide and silicon carbide, the membrane is chosen from the group consisting of silicon, silicon nitride, silicon carbide, diamond, and aluminum oxide, and the mask is chosen from the group consisting of tungsten and tantalum silicon nitride.
 18. The method of claim 16 in which the major surfaces of the filled-windowed substrate are planarized and made parallel before the deposition of the membrane layer.
 19. The method of claim 4 in which the mask support structure is formed by the steps of: forming in a substrate a set of windows spaced apart by major strut portions of the substrate; forming a plurality of spaced apart grooves in the major strut portions of the substrate; and placing one of a plurality of minor strut elongated strips in each of grooves.
 20. A method for forming a mask assembly comprising the steps of: forming by use of a mold a support structure that defines an array of windows arranged in rows and columns; filling the openings with a temporary fill; forming over the support structure a membrane layer; forming over the membrane layer a patterned mask; and removing the temporary fill.
 21. The method of claim 20 wherein the support structure comprises major struts which are orthogonal to minor struts with the minor struts being placed in the mold prior to forming of the major struts and a frame which supports the major and minor struts.
 22. A method of forming a mask support structure comprising the steps of: forming in a substrate a first set of spaced apart windows; filling the first set of windows with a temporary fill; forming in the substrate a second set of windows located in portions of the substrate between the first set of filled windows; and filling the second set of windows with a temporary fill.
 23. A method of forming a mask support structure comprising the steps of: placing in a mold which is shaped to facilitate the formation of a support structure a plurality of parallel minor struts; forming in the mold a mask support structure that comprises a frame and plurality of major struts that are orthogonal and attached to the minor struts with the major and minor struts defining a plurality of windows arranged in a two dimensional array of rows and columns; and filling the windows with a temporary fill.
 24. The mask assembly formed by the method of claim
 1. 25. The mask assembly formed by the method of claim
 2. 26. The mask assembly formed by the method of claim
 3. 27. The mask assembly formed by the method of claim
 4. 28. The mask assembly formed by the method of claim
 5. 20. The mask assembly formed by the method of claim
 6. 30. The mask assembly formed by the method of claim
 7. 31. The mask assembly formed by the method of claim
 8. 32. The mask assembly formed by the method of claim
 9. 33. The mask assembly formed by the method of claim
 10. 34. The mask assembly formed by the method of claim
 11. 35. The mask assembly formed by the method of claim
 12. 36. The mask assembly formed by the method of claim
 13. 37. The mask assembly formed by the method of claim
 14. 38. The mask assembly formed by the method of claim
 15. 38. The mask assembly formed by the method of claim
 16. 40. The mask assembly formed by the method of claim
 17. 41. The mask assembly formed by the method of claim
 18. 42. The mask assembly formed by the method of claim
 19. 43. The mask assembly formed by the method of claim
 20. 44. The mask assembly formed by the method of claim 21
 45. The mask support structure formed by the method of claim
 22. 46. The mask support structure formed by the method of claim
 23. 